1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier and a multi-functional peripheral, and particularly relates to an image forming apparatus provided with an optical scanning device which scans a scanning object (photoreceptor drum) with a light beam.
2. Description of the Related Art
An image forming apparatus which scans a photoreceptor by a laser beam to expose has been known as an electrophotographic image forming apparatus. As a mechanism for scanning by a laser beam, it is common to scan the photoreceptor by reflecting a laser beam emitted from a laser light source provided at a fixed position by a polygon mirror and deflecting the laser beam in a predetermined direction according to variation of a reflection angle associated with a rotation of the polygon mirror.
An image forming apparatus which performs image formation using a multi-laser beam for high-speed writing and high accuracy has been known for an image forming apparatus using such a laser scanning optical system. Further, an image forming apparatus which has a plurality of speed ranges (the number of recording sheets that are printable per minute) has been known.
As to the image forming apparatus provided with the scanning optical system by a multi-laser beam, for example, Japanese Laid-Open Patent Publication No. 2001-228382 discloses a method of adjusting a multibeam light source unit which aims to easily and rapidly perform mounting adjustment of a multibeam laser element in the multibeam light source unit. Here, the multibeam light source unit provided with the multibeam laser element with a plurality of light emitting points that are designed to locate on a virtual straight line that is defined by a notch formed on a stem, and a collimator lens which converts a laser beam emitted from the light emitting point into a parallel pencil are arranged in the scanning optical system with a virtual straight line as a reference. Then, in this case, the mounting adjustment of the multibeam laser element is performed by rotating it around an axis corresponding to an optical axis of the scanning optical system while observing the multibeam element magnified by an enlarging optical device so that displacement which occurs between a direction in which a reference straight line which is planned to be present on the design in the scanning optical system extends and an arrangement direction of the plurality of light emitting points disappears and the both directions are aligned.
Furthermore, Japanese Laid-Open Patent Publication No. 2007-233235 discloses an optical scanning apparatus which aims to form a high quality image without unevenness of concentration, color deviation and color change by adjusting an interval between scanning lines to be even for a scanning frequency and a beam spot interval in a sub-scanning direction of an oscillating mirror that are arbitrarily set. The optical scanning apparatus is provided with light source means having a plurality of light emitting sources, light source driving means which modulates each light emitting source according to pixel information, an oscillating mirror which is supported by a torsion beam as a rotation axis and reciprocates to scan on a surface to be scanned by collectively deflecting a light beam from each light emitting source, and an image formation optical system which forms an image of the light beam from each light emitting source to have a predetermined beam spot interval in the sub-scanning direction on the surface to be scanned. The optical scanning apparatus further includes oscillating mirror driving means which sets a scanning frequency according to a resonance frequency of the oscillating mirror, and pitch adjusting means which adjusts the beam spot interval according to the set scanning frequency of the oscillating mirror.
As described above, the image forming apparatus which performs image formation using a multi-laser beam and has a plurality of speed ranges in one model has been known.
Conventionally, such an image forming apparatus is provided with a skating bit to prevent conversion of the speed range. The skating bit is a data bit which is output from each substrate incorporated into the image forming apparatus for preventing inappropriate conversion, and to notify a CPU for which model the substrate from which the data bit is output is used. The skating bit is output to the CPU as a signal by a voltage value or the like from a predetermined signal line. For example, a skating bit which is different for each of target models is output from the substrate by differentiating an electric hardware configuration for each target model by connecting a predetermined line of the substrate to the ground, by connecting the line to a supply line of a predetermined voltage (for example, 5 V), and the like. With this skating bit, the CPU finds for which model a substrate is used.
The image forming apparatus stores software to control a recording sheet conveyance speed according to each model and information on writing operation timing in one flash ROM to be held, and the CPU uses them by switching the operation timing according to a skating bit output from each substrate.
Some models of image forming apparatuses change an interval between recording sheets without changing a processing speed (speed of a photoreceptor drum) in order to change the speed range shown by, for example, the number of sheets on which image formation is possible to be performed per minute. When the interval between the recording sheets is made smaller, the speed range becomes faster, and when the interval between the recording sheets is made larger, the speed range becomes slower. Thereby, models for a low-speed machine and a high-speed machine are able to be manufactured with basically a same apparatus configuration so as to contribute to expansion of development of the models on the business and the like.
For example, in a case where a malicious user who purchased an image forming apparatus with a low speed range converts a part corresponding to the skating bit of the substrate of the image forming apparatus into that for a high-speed machine from that for the low-speed machine, the image forming apparatus is able to be operated as the high-speed machine.
In a flash ROM of the image forming apparatus, the software and the information on the operation timing to control the conveyance timing of the recording sheet according to all the speed ranges for the high-speed machine and for the low-speed machine are stored as described above, and the CPU of the image forming apparatus thus operates for the high-speed machine when skating bit information for the high-speed machine is obtained from the substrate, while operates for the low-speed machine when the skating bit information for the low-speed machine is obtained. In a case where a malicious user converts the part corresponding to the skating bit of the model for the low-speed machine into that for the high-speed machine, the skating bit for the high-speed machine is output from the substrate and the image forming apparatus provided for the low-speed machine comes to operate as the high-speed machine. Thereby, the low-speed machine is able to be converted into the high-speed machine. This is a so-called state where the skating is discovered.
The conversion of the speed range as described above is an action contrary to an intention of an entity manufacturing and providing image forming apparatuses, and further, allowing the speed range to be converted means that it is possible to modify an inexpensive image forming apparatus for the low-speed machine to the high-speed machine easily, which have been a problem also from an aspect of business profitability of the image forming apparatuses.
The invention of the above-described Japanese Laid-Open Patent Publication No. 2007-233235 adjusts the beam spot interval in the sub-scanning direction according to the variation of the resonance frequencies of the oscillating mirror. Moreover, the invention of the cited reference 1 is made to easily perform the mounting adjustment of the multibeam laser element. These inventions are neither for preventing the conversion of the low-speed machine into the high-speed machine as described above, nor for adjusting the arrangement of the laser element so as to be differentiated between the low-speed machine and the high-speed machine as the present invention.